System and method for controlling a transmitter frequency

ABSTRACT

A system and method for controlling the frequency of a signal being transmitted wherein the frequency of the signal received at the transmitter site is error corrected before being broadcast. The error in the frequency may be developed and applied to the intermediate frequency upconversion and/or the channel upconversion to reduce or eliminate the effects of the error. In the system and method, a pilot frequency signal can be added to the signal to be broadcast after the signal to be broadcast has been shaped and filtered.

BACKGROUND OF THE INVENTION

This application claims the benefit of U.S. Provisional Application No.60/042,932 filed Apr. 4, 1997.

This application is directed generally to circuits and systems forcommunicating analog and digital signals and, in particular, toelectronic circuits and systems for generating and communicating digitaltelevision signals.

For many years commercial television signals have been broadcast using aformat known as NTSC. With the advent of high definition television andthe increasingly occupied radio spectrum, it has been determined thattelevision broadcasters will provide a simulcast of both NTSC signalsand Digital television (“DTV”) signals so that the large installed baseof NTSC receivers and the newly installed DTV receivers will besimultaneously able to enjoy the same broadcasts. Thus, simultaneousbroadcasting will involve the simultaneous transmission of identicalprograms encoded in two different formats over respective televisionchannels. Both the NTSC and the DTV channels have been defined as being6 MHZ in bandwidth.

Simultaneous broadcasting in both NTSC and DTV formats will involve theuse of two different channels. Because the available spectrum for theadditional channels is quite limited, it has been proposed that theadditional channels be allocated in channels which are nearby to otherexisting channels. Where both NTSC and DTV signals are being transmittedover nearby channels, there exists a considerable problem of co-channelinterference where the NTSC and DTV signals-interfere with each other.The interference problem between nearby channels will be worsened ifsignals carried on the DTV and/or NTSC waveforms are not preciselylocated and are not kept within their assigned channel bandwidth.

In proposed DTV systems, and as shown in FIG. 1, a signal bearing videoand audio data may be sent from a signal source, such as a televisionstudio to a RF/Transmission site which may not be co-located therewith.In such a system, the video and audio data signals may be transmitted(or transported) from the studio to the RF/Transmission site usingconventional communications techniques, such as microwave links. Thissignal, often referred to as the Transport Signal, will contain both thedata and the clock for the data, either separately or by the use of aself-clocking coding such as Manchester coding. Conventionally, theRF/Transmission facility would use the data clock for the purpose ofrecovering the data at the Transmission facility. Thus, if the Transferlayer clock is inaccurate or drifts, the effect of the inaccuracieswould work their way into the signal which is eventually broadcast fromthe Transmission facility. Given the importance of maintaining accuratebroadcast frequency, the frequency errors caused by inaccurate clocks atthe Transport layer (often the studio) may be unacceptably high. Asdescribed below, in one embodiment of the present invention, theinaccuracies in the data clock can be removed or compensated by thesystem utilized at the RF/Transmission site so that the signal which isbroadcast has stable frequency components.

As described below with respect to another embodiment of the presentinvention, the problems of co-channel and cross channel interference maybe based, at least in part, in differences between the clocks used togenerate or transmit the video and/or audio signals from theirgeneration to their broadcast. Such interference may be reduced oreliminated by the use of a common clock to serve as the base fordifferent portions of the signal generation and broadcast systemsdescribed herein.

Accordingly, it is an object of the present invention to provide a novelcircuit and system for generating a digital television signal in whichthe frequency based components are precisely generated and/ormaintained.

It is another object of the present invention to provide a novel circuitand system for compensating and/or adjusting for inaccuracies in thedata clock prior to broadcasting of the television signal.

It is yet another object of the present invention to provide a novelcircuit and system that can operate on two asynchronous clocks withoutsynchronous handshaking.

It is further an object of the present invention to provide a novelcircuit and system for adding a pilot signal to a television signal.

It is still another object of the present invention to provide a novelcircuit and system for error handling in a television signal.

It is yet still an object of the present invention to provide a novelcircuit and system for television transmissions where the data signalhas its dependency on the frequency of the digital data rate removed andthe frequency corrected to its nominal condition.

It is further still an object of the present invention to provide novelcircuits and systems for trellis encoding and novel multi-band antennasfor a television system.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the art to which theinvention pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a prior art digital televisionsystem.

FIG. 2 is a functional block diagram of a circuit in the RP transmissionsystem of the present invention.

FIG. 3 is a functional block diagram of the RF transmission system ofthe present invention.

FIG. 4 is an amplitude versus frequency plot for a DTV signal asproduced in the present invention.

FIG. 5 is a functional block diagram of a sync confidence countercircuit of the present invention.

FIG. 6 is a functional block diagram of an interleaving circuit of theprior art.

FIG. 7 is a functional block diagram of an interleaver buffer circuit ofthe present invention.

FIG. 8 is a functional block diagram of a rate conversion circuit of thepresent invention.

FIG. 9 is a functional block diagram of a first embodiment of a controlcircuit for a trellis encoder of the present invention.

FIG. 10 is a functional block diagram of a first embodiment of a trellisencoder of the present invention.

FIG. 11 is a functional block diagram of a second embodiment of atrellis encoder of the present invention.

FIG. 11A is a functional block diagram of another embodiment of atrellis encoder of the present invention.

FIG. 12 is a series of plots showing the amplitude and frequencyrelationship of a theoretical corrected non-linear amplifier and itsrelated signals.

FIG. 13 is a series of plots showing the frequency domain for the seriesof plots of FIG. 12.

FIG. 14 is a plot of the phase and amplitude responses associated withthe input and output circuits of a typical non-linear amplifier.

FIG. 15 is a series of plots showing the frequency domain of a typicalcorrected non-linear amplifier and its related signals.

FIG. 16 is a functional block diagram of an improved corrector circuitin accordance with one aspect of the present invention.

FIG. 17 is a functional block diagram of a second embodiment of thecorrector circuit of FIG. 16, particularly for use in an intermediatefrequency compensation circuit.

FIG. 18 is a functional block diagram of a studio site using an aspectof the present invention.

FIG. 19 is a functional block diagram of a transmitter site using anaspect of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

As defined by the Advanced Television Standards Committee (“ATSC”)Digital Television Standard, dated Sep. 16, 1995, which is incorporatedherein by reference, a basic DTV system 100 block diagram consists ofthree sections, as shown in FIG. 1, including a source coding andcompression section 102, a service multiplex and transport section 104,and a RF/Transmission System 106. The source coding and compressionsection 102 receives a video signal 108 and an audio signal 110 andencodes these signals, respectively, into digital data streams. Theencoding may include bit rate reduction methods and compressiontechniques known and appropriate for video and audio data.

With continued reference to FIG. 1, the encoded video and audio data maybe provided to the service multiplex and transport section 104 alongwith ancillary data signals 112 and control data signals 114. Theancillary signals 112 and control signals 114 may include control data,conditional access control data and data associated with the audio andvideo services, such as closed captioning. Generally the video signalsmay be compressed using a MPEG-2 video stream syntax and the audiosignals may be compressed using the Digital Audio Compression (AC-3)Standard.

In the service multiplex and transport section 104, the compressed datastream may be divided into packets of information and means foridentifying each packet or packet type may be appended. In the process,the video data stream, audio stream and ancillary data stream packetsmay be multiplexed into a single data stream, as is well known in theart. The packeted data may be transported by the MPEG-2 transport systemfor the packetizing and multiplexing of video, audio and data signalsfor digital broadcast systems.

In the RF/Transmission section 106 the packeted data is channel codedand modulated. A channel coder 120 may modify the data stream and addadditional information that can be used by the receiver to reconstructthe data from a received signal which has been affected by typicaltransmission interference sources.

The modulation 122 uses the digital data stream to modulate thetransmitted signal. In a DTV standard, the modulation can use an 8 VSBmodulation scheme. The modulated signal may be amplified and applied toan antenna 124 for broadcast in a conventional fashion.

With reference now to FIG. 2, a RF/Transmission system in accordancewith the present invention may receive a data and clock signal 10 from atransport section as shown in the system of FIG. 1. The data and clocksignal may be applied to a data recovery circuit 202 which extracts thedata from the transport format, and if necessary extracts the data clockfrom the same signal. The data signal may be applied to a randomizer 204which alters the order of the data within the data stream in accordancewith the DTV standards. The randomizer 204 may operate based on agenerator polynomial and a pseudo random binary sequence.

The signal output from the randomizer 204 may be applied to a ReedSolomon encoder 206 which uses the Reed Solomon encoding technique toprovide packets which have been encoded in a predetermined manner, knownto the receiver of the signal. The encoded packets are applied to aninterleaver 208 which performs a convolutional interleaving on the datain accordance with a predetermined scheme. In the present invention, andas explained further below, the Reed Solomon encoder 206 writes to adata buffer (not shown) at the transport layer data rate, and theinterleaver 208 reads the data buffer at the transmission layer datarate. This allows the system to operate on two asynchronous clockswithout synchronous handshaking.

The signal output from the interleaver 208 may be applied to a trellisencoder 210. In a DTV system, the trellis encoder employs a ⅔ ratetrellis code with one unencoded bit. In the DTV system, one input bit isencoded into two output bits using a ½ rate convolutional code while theother input bit is precoded. In accordance with the DTV specification,the signalling waveform used with the trellis code is an 8-level (3 bit)one dimensional constellation. The signal output from the trellisencoder 210 may be applied to a synch insert circuit 212 which insertsthe Data Segment Sync and Data Field Sync at the appropriate locationswithin the digital data stream.

The digital data stream output from the sync insert circuit 212 may beapplied to a Nyquist filter 214 to limit the spectrum of the encodeddata stream to the bandwidth of the transmission system (6 Mhz for thestandard DTV system). The filtering function in the present inventioncomprises the steps of: 1) interpolating and upconverting the signal; 2)correcting the signal for non-linearities; 3) equalizing existingfrequency response imperfections with an adaptive filter; and 4)downconverting and storing the digital sample.

After the Nyquist filter 214 (and related pulse shaping), a pilot signalmay be added. Unlike the prior art, the pilot signal insertion in thepresent invention saves on circuitry by performing the pilot signalinsertion after the pulse shaping by the Nyquist filter. The digitalsignal provided by the Nyquist Filter 214 to which the pilot has beenadded is converted to an analog signal by a digital to analog converter216 and applied to one of the inputs of an IF mixer 30. The other inputsignal to the IF mixer 30 is an IF carrier frequency which has beenderived from an IF Frequency Phase Lock Loop circuit 218. The IF signalgenerated by the IF mixer is applied to an IF Circuit 40 which filtersthe IF signal and applies the output signal to one of the inputs of achannel mixer 50. The other input of the channel mixer 50 is a channelfrequency which has been derived from a UHF Phase Lock Loop 70. In oneembodiment of the present invention, the IFF PLL 218 and the UHF PLL 70may generate their respective signals from a common clock reference 190.The clock reference 190 may base its clocking signal on an externallyderived signal (EXT REF) or on an appropriate clocking signal developedlocally or from the data received by the exciter.

The signal output from the channel mixer 50 may be applied to a bandpass filter 60 in a conventional fashion to develop a filtered signalwhich may be amplified sufficiently and provided to an antenna 220 forbroadcasting. With continued reference to FIG. 2, the signal input tothe data recovery circuit 202 may be either separate data and clocksignals or a single embedded clock signal (such as from the use ofManchester encoding). The clock recovery circuit may use a standardphase lock loop to lock onto the clock embedded in the received data.Once the clock is recovered, it may be used to drive the system timing.The data recovery circuit 202 may also include a frame synchronizer tolocate the sync signals within the signal sent from the transportsection (of FIG. 1) and to align the received data streams into bytesfor processing.

With continued reference to FIG. 2, in the standard DTV system, the ReedSolomon encoder requires 20 modulo 256 multiplies and 20 exclusive-or(XOR) adds for each data byte received from the transport section. Atthe high data rates used in the DTV system, the encoding requires aprocessor capable of 97 million operations per second. In one embodimentof the present invention, the Reed Solomon encoder may be implemented toachieve such a high data rate in a Field Programmable Gate Array inconjunction with a look-up table.

The Nyquist filter 214 may be implemented as a root-raised-cosine filterwith an alpha factor of 0.1152. The filter may be implemented as aFinite Impulse Response filter. In a preferred embodiment, the filter isa complex filter to handle the vestigial sideband function.

With reference now to FIG. 2 and FIG. 3 (in which common elements havebeen provided with common reference numerals), in an embodiment of thepresent invention, the mixers 30, 50 of FIG. 2 and their associated PLLs218, 70 may be implemented in a circuit in which the modulator 20portion of the circuit of FIG. 2 is simplified as a single block,modulator 20, of FIG. 3. A data signal 10 applied to the modulator 20generates a clock signal 302 and an encoded data signal 21 (in analogformat). Because both the clock signal 302 and the encoded data signal21 were originally generated from a digital data stream, the resultingfrequencies of these signals are directly proportional to the digitaldata rate. If these signals were used to develop the signal which is tobe transmitted, the transmission signal will be locked to the frequencyof the data source. In a situation, as described above, in which theTransmission layer is remote from the Transport layer (and may not evenbe under common control), a broadcaster transmitting a signal based on aremote data source in such a situation is not in control of thebroadcast frequency to the rigid specification required of suchbroadcasters. In addition, any frequency offsets to the transmittedsignals required by the regulating authorities, such as the FederalCommunications Commission, for co-channel or adjacent channelinterference problems cannot be performed without changing the originaldata rates. As explained below, in one aspect of the present invention,the dependency of the transmission frequency on the digital data rate isremoved by the circuit shown in FIG. 3. This circuit also provides ameans for making precision frequency offsets which may be required.

With reference now to FIG. 3 and FIG. 4, the modulator 20 generates ananalog modulated frequency signal of encoded data 21. In a standard DTVsignal, the encoded data signal 21 may be in the format of an 8VSBsignal such as specified for DTV. It will be appreciated by thoseskilled in the art that the encoded data signal 21 could be any othermodulated signal, such as an NTSC signal, an FM signal, an AM signaland/or a SSB signal, etc. For the situation of an 8 VSB signal, thesignal will have a form as represented in FIG. 4, in which the signalhas a nominal center frequency at 10.76 MHz and has a VSB pilot signalnominally at 8.06 Mhz. Normally, the encoded data signal 21 is at afrequency considerably below the frequency at which the signal is to bebroadcast and must be upconverted to the broadcast frequency. Theencoded data signal 21 is first applied to a frequency translator 30(such as a mixer) to produce an IF signal 31. In one embodiment of thepresent invention, a signal having a nominal frequency of 54.76 MHz isapplied to the frequency translator to produce the IF signal 31. The IFsignal 31 is then applied to a second frequency translator 50 (such as amixer) to upconvert the IF signal to the desired channel frequency. Thesignal output from the second frequency translator 50 may be applied toa load, after amplification as desired.

As explained further below, the frequencies used by the frequencytranslator 30 to upconvert the encoded data signal is derived from areference frequency 190 as modified by the data clock 302. In this way,the encoded data signal has its dependence on the frequency of thedigital data rate removed and the frequency of the signal is correctedto at or near its nominal condition.

With continued reference to FIG. 3, the digital modulator 20 alsoproduces the digital data clock 302 which is applied to a programmablefrequency divider 180 which is 1 to 1 proportional to the carrier orpilot frequency of the digital modulator 20. Thus, the signal 181 outputfrom the frequency divider 180 for a DTV system is nominally at 8.06MHz. The signal 181 is applied to a third frequency translator 150.

The reference frequency 190 is applied to a reference divider 80, phasedetector 90, loop filter 92, and a Voltage Controlled Oscillator (“VCO”)94. The reference signal 191 is applied to a Numerically ControlledOscillator (“NCO”) 120. The signals output from both the VCO 94 and theNCO 120 are applied to a fourth frequency translator 130. The outputsignal from the fourth frequency translator 130 is band pass filtered(“BPF” 140) and applied to a second input terminal of the thirdfrequency translator 150. The output signal from the third frequencytranslator 150 is band pass filtered by a second BPF 70 and is dividedby a second programmable frequency divider 98 before being applied tothe phase detector 90.

In operation, the frequency of the NCO 110 is controlled by thereference frequency 190. The reference frequency may be locallygenerated or may be received from an external circuit or device (notshown). The reference frequency may be applied to other, similarcircuits so that multiple digitally-generated signals may be frequencylocked to the same reference.

The NCO 120 should be capable of providing frequency resolution to theextent required by the IF signal 31. The output of the NCO 120 issubtracted from the output of the VCO 94 in the fourth frequencytranslator 130. The BPF 140 is used to filter out any undesired outputfrom the fourth frequency translator 130.

In operation the third frequency translator 150 subtracts the outputsignal of the programmable divider 180 from the output of the first BPF140. The second BPF 70 removes any undesired output from the thirdfrequency translator 150 and applies this difference frequency to thesecond programmable frequency divider 98. For the nominal signalcondition, the second programmable divider 98 and the reference divider80 are set such that their output frequencies are the same. The outputsof these two dividers 98, 80 are provided to the phase detector 90.Thus, the output of the phase detector 90 is proportional to the phasedifference between its two inputs and is applied to the loop filter 92.The loop filter 92 converts the output of the phase detector 90 into avoltage which is proportional to the phase difference and applies thisvoltage to the VCO 94. As a result, the VCO 94 is frequency locked tothe reference frequency and not to the original digital rate.

In operation with an input data signal at an off-nominal data rate, thecircuit of FIG. 3 removes the effect of the off-nominal rate. Forexample, if the clock rate of the incoming data 10 is off nominal by“delta” the frequency of the signal 302 will equal 10.76 MHz+delta andof the signal 181 will equal 8.06 MHz+delta. The delta is propagatedthrough the reference frequency components and ultimately through theVCO 94 so that the signal used to upconvert at the first frequencytranslator 30 is also offset from the nominal by delta. Since the datasignal at signal 21 is also offset by delta, the first frequencytranslator 30 will subtract out the two delta components leaving thesignal 31 free of the delta offset.

In the DTV standard transport protocol, the sync signal (a byte) is nota unique number. Accordingly, a false indication of synchronization maybe given if any eight contiguous bits of data are identical to the syncbyte. The present system overcomes the effect of this false sync byusing a confidence counter. The confidence counter counts the number ofsync bytes continuously. With reference to FIG. 2, if a sync byte is notdetected by the data recovery circuit 202 when expected, confidence ofhaving sync lock is reduced and the confidence counter is decremented.If several frames occur without a sync byte being detected at a specificbyte location within the data stream, the frame synchronizer willindicate that the system is out of sync lock and can automatically begina new frame search and acquire operation.

With reference now to FIG. 5, in one embodiment of the presentinvention, a frame and bit synchronizer using a confidence counter mayreceive data as received from the transport layer in a serial form(SERIAL DATA), the serial data may be converted to a parallel form (suchas an 8 bit parallel data word) by a serial to parallel converter 750and provided to a sync detector 752. The sync detector 752 compares thedata word to a predetermined sync word and a signal sync indicatingwhether sync was found is provided to a confidence counter 754 and toAND gate 756. The confidence counter 754 increments a counter if syncwas found where expected and decrements the counter if sync was notfound where expected. A sync is “expected” exactly one frame from thelast detected sync word. Thus, a modulo counter 758 counts the number ofwords encountered since the last sync detection and enables theconfidence counter 754 at that word at which sync is expected. If theconfidence counter 754 reaches a predetermined value, the frame isconsidered in sync and a SYNC LOCK signal is sent to other elements inthe circuit and/or displayed for the system operator. The AND gate 756in the presence of a 0 count in the confidence detector 754, provides aRST pulse to modulo counter 758. Thus, if the confidence counter 754 isdecremented to zero, sync is considered to be lost causing the SYNC LOCKsignal to be removed and resetting the frame search initiation circuitof the modulo counter 758 to begin the search for sync anew (i.e.,starting a new frame search and acquire mode).

With continued reference to FIG. 5, the modulo counter 758 also providesa modulo 8 count of serial bits coming into the serial to parallelconverter 750. In this way, the converter 750 remains bit aligned withthe rest of the sync detection circuit. At the end of each modulo 8count, the data word is provided to the data randomizer along with beingprovided to the sync detector. The data randomizer and other elements ofthe circuit can use the SYNC LOCK signal to determine whether the databyte is valid. The confidence counter can readily be modified to obtainany degree of confidence that sync lock has been achieved; generally,the higher confidence required necessitating a longer sync detectiontime. In addition, the loss of sync lock need not wait until theconfidence counter returns to zero (e.q., five correct sync detectionscould be required to obtain lock but only three consecutive failures todetect sync could cause a loss of sync.) Similarly, the number ofsuccessful sync detections can be capped so that once sync has beenlocked, the confidence counter will not increment past the locking value(but will decrement if an expected sync is not found).

At the startup after the loss of input sync, the confidence countercircuit is set to begin an acquisition mode. In the acquisition mode,the serial data bits are clocked serially into the serial to parallelconverter 750 which may take the form of an 8-bit buffer. Thereupon, ateach clock cycle, the 8-bit data word is checked for the presence of thesync byte. If the sync byte is detected, a count enable bit is set andthe circuit enters a confidence testing mode. Whenever the count enablebit is set, the frame counter circuit of the modulo 1504 counter beginscounting until the next 1504 bits (188 words of 8 bits each, or 1 frame)have been transmitted. At this time, the sync is checked again and ifpresent the process is repeated. If the sync byte is not present, thecount enable bit is reset and the acquisition mode is restarted. If apredetermined number of consecutive frame syncs are detected, the SYNCLOCK signal is set and the circuit enters a Frame Lock mode. thereafter,normal operation begins and the frame sync is checked continually toensure reliable transmission. If frame sync is not found for apredetermined number of frames, the circuit will switch back to theacquisition mode, restarting the entire process.

Note that the confidence counter circuit of the present invention may beutilized in both the transmission of the DTV signal and in receiversattempting to receive and display DTV or other digitally framed signals.Such receivers are faced with a similar problem of establishing andmaintaining frame sync as described above and the circuit of the presentinvention will also be useful to such receivers.

As mentioned above, in one aspect of the present invention, thefrequency of the signal used in the transport layer can be madeindependent of the frequency of the signal in the transmission layer bya data interleaver. It was known in the prior art to interleave data ina manner in which the frequency of the data was changed. For example,and with reference now to FIG. 6, a signal from the transport layer,carrying the data at a 19.39 MHz rate, would typically be provided to aReed Solomon encoder for encoding in a conventional fashion. The dataexiting the Reed Solomon encoder 550 would be provided to amicroprocessor 552 which would interleave the encoded data and provideit, at an arbitrary rate, usually in a burst mode, to a Re-Timer Buffer554. The Re-Timer Buffer 554 will typically buffer the burst mode datafrom the microprocessor and output a signal at a specific rate asgoverned by its clock, usually the standard 10.76 MHz rate. Often inprior art systems, the 10.76 MHz clock signal provided to the Re-TimerBuffer 554 would be generated from a PLL 556 being driven from the 19.39MHz clock (often obtained from the data).

In another aspect of the present invention, the burst modemicroprocessor of the prior art may be eliminated with a suitableinterleaver buffer. With reference to FIG. 7, a Reed Solomon encoder 206may receive the data at the frequency of the transport layer, 19.39 MHz,and provide the encoded data to a data interleaver buffer 208. The datais read out of the interleaver buffer 208 at a rate of 43.04 MHz andprovided to the trellis encoder 210 which outputs the trellis encodeddata at the 10.76 MHz rate generally associated with the transmissionlayer. Note that the rate converted 43.04 MHz clock is available to thetrellis encoder 210 and therefor can be used to time the extraction ofthe data from the interleaver buffer 208 and for the trellis encoding.The clocks to the trellis encoder are provided by a rate converter 650which converts its incoming 19.39 MHz clock signal by a ratio of313*207*832/312/188/828 to develop a 43.04 MHz clock signal. Theresulting clock signal can be decreased or divided by a factor of 4 toprovide a 10.76 MHz clock and may be provided to other elements in thetransmission layer. Note that in this aspect of the present invention,the digital elements to either side (input or output) of the interleaverbuffer 208 are made more simple than in the prior art because theseelements can operate in a continuous mode and do not have to be sizedfor burst mode operations.

With continued reference to FIG. 7, the 19.39 MHz clock may be obtainedfrom the data (as received or as corrected in another aspect of thepresent invention) or from a local oscillator. The use of a localoscillator ensures that the trellis encoder will continue to operatedespite the loss of the transport signal or the loss of the clock in thetransport signal. Indeed, the local clock can be used to drive the ReedSolomon encoder in the event of a loss of signal from the transportlayer (the encoded signal). Because at least one promulgated standardrequires that the transmission layer continue to transmit a signal(generally a pseudorandom signal) even if the originally encoded signalis lost, keeping the Reed Solomon encoder in operation provides thenecessary data signals to maintain a transmission signal.

In yet another embodiment of the present invention, if a sufficientlyrobust Digital Signal Processor (“DSP”) is used, all of the Reed Solomonencoding, data interleaving and trellis encoding and frequencyconversion can be accomplished in a chain of one or more suitablyprogrammed DSPs which have been provided with the desired clock(s).

With reference now to FIG. 8, a rate converter circuit in accordancewith one aspect of the present invention may include a Divide By 141circuit 570 which receives a clock signal (such as a 19.39 MHz signal)and provides the frequency divided signal to a phase detect circuit 572and, subsequently, to a loop filter 574 which drives a thermallycompensated voltage controlled oscillator 576. The output of the TXCO576 is provided to a buffer 578 and is fed back to a Divide By 313circuit 580 which is also connected to the phase detector 572. The TXCO576 has a center frequency at the desired output frequency (e.g., 43.04MHz) and has a rail at +/−3 Hz which is the maximum permitted deviationof the carrier frequency in a standard DTV system.

In operation, so long as a clock frequency is present at the input tothe circuit and the frequency of that signal is within the desiredlimits of the system, the generated 43.04 MHz clock may be permitted totrack the input signal. If the input signal varies in frequency outsidethe limits, the TXCO 576 will not go outside its rails and will limitmaintain the output frequency within the required specification.

With continued reference to FIG. 8, the useability of the input signalto serve as the basis for the output signal may be indicated by anexternal signal, such as the SYNC LOCK signal described herein inassociation with the confidence counter circuit. When the system is outof sync, a gate 582 can provide a voltage signal which will drive theTXCO 576 to the desired output frequency. If desired, the magnitude ofthe voltage signal can be set by the operator of the system, such asthrough a user-settable potentiometer (not shown).

With reference now to FIG. 9, a trellis encoder which may be used in thepresent invention receives a stream of digital data to be encoded (DATAIN). The digital data stream is applied to a FIFO Buffer 52 and aStaging/Timer Circuit 54 to align the data in time with the remainder ofthe circuit. At the appropriate time, the data are applied to a nibbleselect circuit 56 which selects the appropriate two bit nibbles from theinput digital data in accordance with the rules of the encoding scheme.In turn, the selected nibbles are encoded by a trellis encoder 58 (whichfor the DTV system supplies 3 bits of output data for each two bits ofdata input to it). The data output from the trellis encoder 58 aresupplied to a multiplexer which multiplexes the data with sync bitsand/or with pseudo random number encoding output of the multiplexer 59is supplied to an output register 59. The operation of the circuit ofFIG. 9 is controlled by control logic 51 which coordinates framing ofthe data, resetting symbol and segment counters 53, 55 as appropriate.The outputs of the counters 53, 55 are provided to an enabler 57 whichprovides pacing signals to the other elements of the circuit so thateach element needing to know the location of the incoming data withinthe frame will have that data available.

In operation, a digital data stream is provided to the trellis encodercircuit and is received by the input buffer and staging elements. Thecontrol logic 51 reviews the incoming data to determine frameboundaries. Once framing has been established, the control logic 51resets the symbol counter 53 and the segment counter 55 to start a newframe. As the data is thereafter received, the symbol and segmentcounters 53, 55 are incremented and cycle themselves in accordance withthe format of the data. The symbol and segment counters 53, 55 drive theenabler 57 which sends the appropriate control/selection signals to theother elements of the circuit.

With continued reference to FIG. 9, guided by the signals from theenabler 57, the nibble select circuit 56 selects the appropriate nibblesfrom the input data stream to be encoded next. In the case of a standardDTV signal, the nibbles selected are done in accordance with the DTVspecification [Annex D, table 2 to the Digital Television Standard],although any other conversion scheme could be implemented. Thereafter,each nibble is encoded in the order selected by the trellis encoder 58.As appropriate for the specified protocol and as guided by the enabler57, the multiplexer alternatively outputs the trellis encoded data orsync data in accordance with the protocol being followed by the system.In the event that an input data signal is lost or loses sync, apseudorandom stream of data bits (“PN”) can be applied by themultiplexer 59 to the output data stream.

With reference now FIG. 11, a nibble select circuit 56 which may be usedin the present invention receives a stream of digital data (DATA IN) ateach of twelve storage registers R1,R12. The stream of input dataarrives serially and may be cycled into successive ones of the storageregisters R1,R12. The twelve registers are connected to one side of afirst multiplexer 62. Under control of a multiplexer control signal (notshown) based on the location of the input data within the frame, thefirst multiplexer 62 presents one of the bytes of input data at itsoutput terminals. A second multiplexer 64 under control of a secondmultiplexer control signal (not shown), receives the selected byte andselects two bits of the data to output. The output bits are applied tothe trellis encoder 66 which encodes the two bits into three bits whichare then passed to the next stage in the transmission system.

Still another embodiment of a trellis encoder circuit in accordance withone aspect of the present invention is shown in FIG. 11A in which aninput data stream (DATA IN) is directed by a first multiplexer 82 (notshown) to twelve parallel registers R1,R12. Once the registers areloaded, nibble select circuits 84 associated with each of the storageregisters R1,R12 will select two data bits from their associated storageregisters R1,R12 and provide the selected bits to an associated trellisencoder 86. Each trellis encoder 86 encodes the received bits inaccordance with the predetermined encoding scheme and provides the threeencoded bits to the input of an output multiplexer 88. The outputmultiplexer selects the encoded bits from the appropriate trellisencoder 86 in accordance with the protocol. Note that the trellisencoder of the embodiment in FIG. 11 requires a depth of 1 whereas thetrellis encoder of FIG. 10 requires a depth of 12 if the trellisencoding scheme of the standard DTV system is being used.

It is well known in the prior art that signal amplifiers often introducenon-linearities into the signals being amplified. To linearize signalswhich are to be amplified by such non-linear amplifiers, it is also wellknown to pre-distort the input signal in a manner which is opposite tothe non-linearity effects expected to be applied by the amplifier. Forexample, and with reference to FIG. 12 (showing the amplitude of thesignal versus frequency), an input signal having the form in FIG. 12(A)will be pre-distorted (or pre-corrected) by a transfer function as shownin FIG. 12(B). After the amplifier applies the amplification effectshown in FIG. 12(C) to the precorrected input signal, the corrected,amplified signal form is obtained, as shown in FIG. 12(D). For furtherexplanation, in FIG. 13 is depicted the same signals and effects asshown in FIG. 12, this set of drawing figures being shown in thefrequency domain.

It has been found that the success of the prior art precorrectionmethods are usually limited to the amplifier's frequency response andthe system band width. Pre-correction products which have been used tocancel the amplifier's distortions are modified in phase and amplitudeby the amplifier's input circuits. These changes in the phase andamplitude limit the cancellation of the distorting products in theamplifier. For example, the input circuits of a typical amplifier oftenhave the amplitude and phase responses shown in FIG. 14 over thefrequency bands of interest. These amplitude and phase changes preventthe precorrection from cancelling all of the unwanted distortion of theamplifier. The resultant signals which are usually obtained in such acircuit are shown in FIG. 15, a frequency domain series of drawings.Note the sidelobes of frequency elements caused by the uncorrecteddistortion.

One means for eliminating the unwanted non-linearities is to insert anamplitude and phase corrector between the prior art pre-correction andthe non-linear amplifier. The corrector is adjusted so that itsamplitude and phase responses are the inverse of the amplitude and phaseresponses of the non-linear amplifier.

With reference to FIG. 16, a circuit which can carry out the improvedamplifier system response may include a non-linear amplifier 170 and itsassociated amplifier input circuits 168 and amplifier output circuits172. A precorrection circuit for amplitude versus input (i.e., alinearity corrector) 164 and for phase versus input (i.e., a phasecorrector) 162 may adjust for non-linearities in a conventional fashionfor the particular amplifier 170 and the frequency ranges of interest. Afrequency response and phase corrector 166 is placed between thelinearity corrector 164 and the amplifier input circuits 168 and isadjusted to compensate for the inputs circuits so that the net result ofthe cascaded pair of elements (corrector 166 and circuits 168) is unity.In this way, the precorrection signal can be fully compensated in thenon-linear amplifier 170.

Note that in the circuit of FIG. 16, the phase corrector 162 precedesthe linearity corrector 164. This is the preferred configuration ofthese correctors because if these correctors are reversed in order, theadjustment of the amplitude pre-correction would cause undesired changesin the phase pre-correction. These undesired changes can be eliminated;however, the adjustment of the circuit to eliminate the effects of theundesired interaction is made more difficult.

The amplifier output circuits 172 of the non-linear amplifier 170 willoften also modify the overall system frequency and phase response. Tocontrol this effect, a frequency response corrector 160 can be added tocorrect the overall system for transparency in frequency and phaseresponse.

The frequency response correctors 160 and 166 may each be realized inpiecewise linear correction circuits which can individually modify asignal's frequency response and group delay (or phase) response over aspecified frequency range. These correctors should be capable of: (1)modifying amplitude versus frequency independently of phase (delay); and(2) modifying phase (delay) versus frequency (so called “group delay”).In a preferred embodiment, the frequency response correctors wouldimplement these stated capabilities independently of each other.

The phase corrector 162 may be realized in a piecewise linear correctioncircuit which can modify a signal's phase transfer function as afunction of the input level and are well known in the art. The linearitycorrector 164 may be realized by a conventional piecewise linearcorrection circuit which can modify a signal's linearity transferfunction as a function of the input level and are also well known in theart. The amplifier used in the circuit of FIG. 16 may be any deviceintended to increase the level of an input signal to a higher level butgenerally has non-linearities or discontinuities in its transferfunction. Such an amplifier may be solid state (such as a bipolar,MOSFET, LDMOS, silicon carbide, etc.) or may be a vacuum tube (such asan IOT, tetrode, klystron, etc.). The amplifier input circuits 168 maytypically include input circuits to the amplifier such as input matchingcircuits, filters, cavities, circulators, etc. The amplifier outputcircuits 172 may typically include output circuits from the amplifiersuch as output matching circuits, filters, cavities, circulators, etc.

In practice, the set of correctors used in this aspect of the presentinvention would preferably be designed starting with the phase corrector162 and linearity corrector 164. Such corrector circuits can be designedconventionally to produce a desired predistorted signal inverselymatching the abnormalities of the amplifier. Having designed thelinearity and phase correctors, the frequency response corrector whichcomes after the linearity corrector can next be designed as it willcorrect for the frequency related errors caused by the amplifier inputcircuits. Finally, the frequency response corrector at the beginning ofthe signal flow can be designed to obviate errors in the overalltransfer function including the amplifier output circuits.

With reference now to FIG. 17, in which elements in common to theelements of FIG. 16 have been provided with a common reference numeral,it is known that pre-correction in prior art systems is oftenaccomplished at an intermediate frequency. The block diagram of thisaspect of the present invention may be modified as shown in FIG. 17where a frequency mixer (or translator) 174 is placed between theprecorrection elements and the non-linear amplifier 170. The mixer 174may be supplied with a signal from a frequency reference 176 totranslate the signal being amplified to a new frequency. The frequencyreference 176 may include an oscillator, tuned circuit, externalreference, or any conventional means or circuit to provide a referencefrequency.

The tight frequency control requirements which are to be levied upon DTVbroadcasters to ensure a minimum of interference with other channelsrequires that special attention be given to the effects ofintermodulation products and frequency stability and control. One systemfor obtaining satisfactory frequency control is illustrated in thefunctional block diagrams of FIGS. 18 and 19, which represent a typicalsite at which the television signals are generated (such as a studio)[FIG. 18] and at which they are broadcast (such as a transmittersite)[FIG. 19]. With reference to FIG. 18, television signals, STUDIOA/V, from multiple and alternative sources may be provided to aconventional MPEG-2 encoder 180 which encodes the signals in aconventional fashion using an external clock reference 182. The clockreference 182 may be any highly stable clock source, particularly aclock source which can be readily received at remote locations, such asa GPS signal. The encoder 180 may provide an internal decoder so as toprovide a monitor signal to a local audio-visual monitor 184. Theencoded television signal(s) are provided to a “Grand Alliance”transport mechanism 186 which will format the incoming MPEG-2 signalinto the proper format needed for the “transport layer” of the GrandAlliance system. Alternatively, the GA transport mechanism may receive(through a selectable switch) a signal from an external MPEG-2 signalgenerator. The transport formatted signal may be provided to amultiplexer 188 which multiplexes the transport formatted signal withother data and provides the multiplexed data to a transmission facility190 for transmission to a broadcast transmitter site. Along a somewhatparallel path, a “standard television” video signal or signals may beconverted to a digital form by an analog to digital converter and thedigitized video signal, along with its corresponding digital audiosignal, may be provided to a NTSC 4/1 compressor 194. The compresseddigital NTSC signal is provided to the multiplexer 188 for multiplexingwith the transport formatted signal and for transmission by thetransmission facility 190. Each of the encoder 180, transport mechanism186, multiplexer 188, A/D 192 and compressor 194 is locked to the commonclock reference 182. If the clock reference is taken from the well-knownGlobal Positioning Satellite system (or any similar available, stablesystem), the studio site can us the 1 MHz reference signal provided bythe satellite system. Thus, both the DTV signal (generated by thetransport mechanism 186) and the digital NTSC signal are locked forefficient digital transmission to the transmitter site. Because the DTVsignal is locked to the common reference clock, multiplexing may be donein a synchronous mode with the digital NTSC signal.

With reference now to FIG. 19, when the multiplexed signal arrives atthe transmitter site 196, it may be demultiplexed by a demultiplexer(which may use any Forward Error Correction signals provided by thetransmission facility 190) and provided as two data streams, DTV andNTSC. The DTV signal may be formatted for DTV broadcast and amplified byan exciter 200, power amplified and upconverted, and ultimatelybroadcast through a suitable antenna 204. The NTSC signal may be decodedto baseband by a NTSC decoder 208 and broadcast through conventionalmeans. The demultiplexer 198, exciter 200, PA & Upconverter 202 and theNTSC decoder 208 may each be locked to a common clock reference 212.

The clock reference 212 at the transmitter site is optimally selected tobe common to the clock reference 182 used at the studio site [FIG. 18].For example, both sites could use the same GPS reference signal. Byusing the same reference signal (such as the GPS 10 MHz signal), the DTVsignal and the NTSC signal are locked to the same stable source. As aresult, the NTSC chroma signal (at 3.58 MHz) is locked to a stablefrequency source which will reduce the likelihood and/or severity ofinterference between the transmitted DTV pilot signal and thetransmitted NTSC chroma signal.

Other stable clock reference signals can be used in common between thestudio site and the transmitter site to achieve the advantages of thepresent invention. For example, if the studio site is sufficientlyproximate to the transmitter site, the STL transmission can beeliminated and the clock signal can be carried by conventional meansbetween the studio and the transmitter.

If desired, fewer than all of the components shown as being related tothe clock reference (CLK REF) in FIGS. 18 and 19 need to be tied to asingle (or related) clock reference and come within the teachings of thepresent invention. The improvements in adjacent and co-channelinterference obtained from the use of this embodiment of the presentinvention can be obtained if only the DTV exciter 200 and thetransmitter (PA and Upconvert 202) use a common clock reference. Inaddition, the advantages of the present invention can be utilized morebroadly by using a common clock reference at plural sites. For example,if plural exciters and transmitters use a common stable reference, suchas a satellite signal, each could generate a NTSC and DTV signal whichwould reduce interference not only with the co-generated televisionsignals but also with other signals generated at other sites whoseassigned channels may have otherwise caused interference from frequencyinaccuracies.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalence, manyvariations and modifications naturally occurring to those of skill inthe art from a perusal hereof.

What is claimed is:
 1. In digital television transmitters including anexciter circuit for receiving digital television signals that includedigital data and clock signals and formatting for transmission andconverting the digital television signals to analog form that includesanalog data and an analog reference signal that is a function of thetiming of the digital clock signal, and an analog power stage forreceiving and amplifying the analog and reference signals that includesat least one frequency up converter stage for mixing a carrier with theanalog and reference signals, a correction circuit for correcting forany deviations in input digital clock signals comprising: a frequencycontrollable source of carrier signals including a reference signal, forproviding carrier and reference signals to the up converter stage formixing with the analog and reference signals, and a clock correctioncircuit adapted for receiving the digital clock signals for detectingany deviation from a standard and applying a correction signal to thefrequency controllable source to shift the frequency of the frequencycontrollable source in a direction to cancel any deviation in the analogreference signals resulting from deviations in the input digital clocksignals.
 2. The correction circuit of claim 1 further comprising: aconversion circuit for converting the digital clock signals into ananalog reference signal having a frequency that is a function of thedigital clock signals and corresponding to the analog reference signalfrom the exciter; and, a comparison circuit for comparing the frequencyof the converted analog reference with a reference signal and with thecarrier reference signal to produce the correction signal to change theoutput frequency of the controllable source in a direction to reduce thecorrection signal.
 3. A frequency controlling circuit for digitaltelevision transmission independent of digital data rate comprising: adigital modulator receiving a data stream, generating an analogmodulated frequency signal, and generating a digital data clock; a firstfrequency translation means receiving the analog modulated frequencysignal and producing an IF signal; and, a second frequency translationmeans upconverting the IF signal to a broadcast frequency, wherein thefirst and second frequency translation means receive a correction signalderived from the digital data clock.
 4. The circuit of claim 3 furthercomprising a programmable frequency divider proportional to a pilotsignal of the digital modulator, receiving the digital data clock fromthe digital modulator, and generating a signal with stable frequencycomponents.
 5. The circuit of claim 4 further comprising a thirdfrequency translator subtracting the output signal from the programmabledivider from the output of the filter.
 6. The circuit of claim 5 whereinthe reference frequency is applied to a voltage controlled oscillatorloop and the reference signal is applied to a numerically controlledoscillator loop for controlling the frequency components.
 7. The circuitof claim 6 wherein the output of the voltage controlled oscillator loopand the numerically controlled oscillator loop is further applied to afourth frequency translator and filtering before transmission.
 8. Thecircuit of claim 7 wherein the output of the third frequency translatoris applied to a second programmable frequency divider and phase detectorbefore transmission.
 9. A method for maintaining the frequency ofdigital television signals to be transmitted by a digital televisiontransmitter within digital television standards to reduce potentialco-channel interference by a television transmitter that is adapted toreceive television signals from a television signal source providingsignals in the form of a digital data stream along with clock signalsinvolved in the synchronization of the digital data stream and fortransmitting the digital data stream as digital television signals, themethod comprising: extracting the clock signals from the data stream;comparing the extracted clock signals to a source of reference clocksignals to generate a difference signal that is a function of the timingbetween the extracted clock signals and the reference clock signals, andusing the difference signal to control the frequency of transmission ofthe digital television transmitter in a manner so as to compensate forany deviation between the timing of the extracted clock signals and thereference clock signals.
 10. The method for maintaining as defined inclaim 9 wherein the digital transmitter includes a power amplifier stagewith at least one up converting stage for modulating a higher frequencyby the digital television signals to be transmitted and wherein: thestep of using the difference signal to control the frequency oftransmission controls the frequency of the higher frequency applied tothe up converter stage in a direction to compensate for any deviationbetween the timing of the extracted clock signals and the referenceclock signals.
 11. The method for maintaining as defined in claim 10wherein the power amplifier stage includes an intermediate frequency upconverter stage and a channel frequency up converter stage and wherein:the step of using the difference signal to control the frequency oftransmission controls the frequency of the intermediate frequencyapplied to the intermediate frequency up converter stage in a directionto compensate for any deviation between the timing of the extractedclock signals and the reference clock signals.
 12. The method formaintaining as defined in claim 9 wherein the digital televisiontransmitter converts the digital data stream into analog signal formrepresentative of the digital signals to be transmitted and a referencesignal is included in the analog signal, and wherein: any differencebetween the extracted clock and the reference clock in the comparingstep is reflected in the reference signal included in the analog signal,and the step of using the difference signal to control the frequency oftransmission controls the frequency of the higher frequency applied tothe up converter stage to change the frequency of the higher frequencyin a direction to correct the reference signal outputted from the atleast one up converter stage.
 13. The circuit of claim 5 wherein theoutput of the third frequency translator is applied to a secondprogrammable frequency divider and phase detector before transmission.14. The method for maintaining as defined in claim 9 wherein the digitaltransmitter includes a power amplifier stage with at least oneintermediate frequency up converter stage for modulating an intermediatefrequency by the digital television signals to be transmitted andwherein: the step of using the difference signal to control thefrequency of transmission controls the frequency of the intermediatefrequency applied to the up converter stage in a direction to compensatefor any deviation between the timing of the extracted clock signals andthe reference clock signals.
 15. A digital television transmittercomprising: a digital modulator for receiving digital input signalsincluding a data portion and a clock portion that comprise televisioninformation signals to be transmitted by the transmitter, the digitalmodulator including a digital formatting circuit for formatting thedigital input signals for transmission by a transmitter, a digital toanalog converter circuit for converting the formatted digital signals toanalog output signals and a circuit for providing the digital clockinput signals as an output; a power amplifier circuit for receiving theanalog output signals, mixing the analog output signals with at leastone carrier signal, and amplifying the mixed carrier signal, the poweramplifier circuit including at least one up converter stage forreceiving a carrier signal including a reference signal; a clockcorrection circuit receiving the digital clock input signal from thedigital modulator for detecting any deviation in the digital clockreference signal and providing a deviation signal as a function of thedegree of deviation detected; a variable carrier signal source forgenerating the carrier signal and the reference signal, responsive tothe deviation signals from the clock correction circuit for changingcarrier frequency in a direction so as to compensate for any deviationin the digital clock input signal.
 16. A transmitter as defined in claim15 wherein: the power amplifier circuit includes two up converterfrequency stages, a first intermediate frequency converter and a secondoutput radio frequency carrier converter; an immediate frequency carriersource and an output frequency carrier source for providing carrierfrequencies to the intermediate and output radio frequency converters,respectively, and the intermediate frequency source provides thereference signal and is responsive to the deviation signal from theclock correction circuit for modifying the analog carrier signals so asto compensate for any deviation in the digital clock input signal.
 17. Atransmitter as defined in claim 15 wherein: any deviation in the inputdigital clock signal is reflected in a reference signal included in theanalog output signal, and the deviation signal from the correctioncircuit changes the frequency of the reference signal in the carrierfrequency in a manner so as to correct the reference outputted from theat least one upconverter.
 18. A digital television transmittercomprising: an exciter circuit adapted to receive a digital data streamcorresponding to digital television signals for formatting the digitaldata stream and for converting the formatted digital data stream intoanalog form in accordance with digital television standards, wherein thedigital data stream received by the exciter circuit includes a clocksignal and the exciter circuit extracts the clock signal from thedigital data stream; a power amplifier coupled to receive the digitaltelevision signals in analog form from the exciter circuit, including atleast one mixer circuit, for modulating the digital television signalsin analog form with a carrier frequency at a designated televisionchannel frequency and for amplifying the modulated carrier signalshaving a bandwidth in accordance with digital television standards; and,a frequency control circuit, for controlling the frequency of thebandwidth of the modulated carrier signals, coupled to apply upconverting signals to one of the mixer circuits, the frequency controlcircuit being coupled to receive the extracted clock signals from theexciter circuit and comparing the extracted clock signals to a standardand to use the difference to control the frequency of up convertingsignals.
 19. A digital television transmitter as defined in claim 18wherein a pilot frequency insertion circuit coupled to the excitercircuit for inserting a pilot frequency into the digital data streamjust prior to conversion of the digital data into analog form.
 20. Adigital television transmitter comprising: an exciter circuit adapted toreceive a digital data stream corresponding to digital televisionsignals for formatting the digital data stream and for converting theformatted digital data stream into analog form in accordance withdigital television standards; a power amplifier coupled to receive thedigital television signals in analog form from the exciter circuit,including at least one mixer circuit, for modulating the digitaltelevision signals in analog form with a carrier frequency at adesignated television channel frequency and for amplifying the modulatedcarrier signals having a bandwidth in accordance with digital televisionstandards; and, wherein the exciter circuit includes a first portionsynchronized to the clock signals of inputted digital data signals andsecond portion under synchronous control of a rate conversion circuitthat is in synchronism with clock signals of the digital data signalsbut is adapted to provide synchronization independent of digital signalswhen the digital signals deviate beyond preset limits.
 21. A digitaltelevision transmitter as defined in claim 20 wherein a pilot frequencyinsertion circuit coupled to the exciter circuit for inserting a pilotfrequency into the digital data stream just prior to conversion of thedigital data into analog form.